UNSEGMENTED WORMS. 197 



"arms"). The lophophore may have a skeletal support which in differ- 

 ent types assumes different shapes (loop, helix, or spiral). A peduncle 

 usually extrudes at the hinge, by means of which the animal attaches itself 

 to foreign objects. The Brachiopods are not colonial. The student is 

 referred to the more extended texts for illustrations of this group of 

 animals. 



231. Notes on Ecology and Distribution. The organ- 

 isms included in this chapter represent the most varied modes 

 of life. The Turbellarians are free animals and may be ter- 

 restrial, fresh-water or marine; the Rotifers are as a rule 

 free-swimming and occur chiefly in fresh water; the Polyzoa 

 are aquatic, attached, colonial forms but lead for the most 

 part an independent existence, or may occasionally be com- 

 mensal with other types of animals; the Brachiopods are 

 marine and may be attached, but are not colonial ; the Trema- 

 todes and Cestodes represent all kinds and degrees of para- 

 sitism. Even if all these classes of animals could be con- 

 sidered akin, their habits of life and their consequent adapta- 

 tions are so various as to produce the greatest range of gen- 

 eral form and special structure. 



If we consider the relatively small number of species of 

 animals in these groups, the species of the Platyhelminthes are 

 among the most widely distributed of the metazoa. This is 

 true both of the free Turbellaria and the parasitic Trematodes 

 and Cestodes. There is probably not a large group of the 

 metazoa which escapes being the host of one or more of these 

 worms at some stage of its life history. The fact of para- 

 sitism, the ability to carry on the life cycle in a series of hosts, 

 and the prevalence of the carnivorous habit among its hosts 

 all help the distribution. The organs more commonly infested 

 by the parasites are the digestive tube, the blood and lymphatic 

 vessels, the coelomic cavity or other organs where the nutritive 

 fluids of the body are abundant. They produce all sorts of 

 disorders from mere functional disturbance (such as digestive 

 disorders and anaemia from the presence of the tape-worm) 

 to the destruction of the tissues of the organs involved. It 

 . is very commonly true that the adult or sexually mature in- 



